1. Field of the Invention
The present invention relates to an extreme ultra violet (EUV) light source apparatus to be used as a light source in exposure equipment.
2. Description of a Related Art
In recent years, as semiconductor processes become finer, photolithography has been making rapid progress to finer fabrication. In the next generation, microfabrication of 100 nm to 70 nm, further, microfabrication of 50 nm or less will be required. Accordingly, in order to fulfill the requirement for microfabrication of 50 nm or less, for example, exposure equipment is expected to be developed by combining an EUV light source generating EUV light with a wavelength of about 13 nm and reduced projection reflective optics.
There are three kinds of light used as an EUV light source: an LPP (laser produced plasma) light source using plasma generating by applying a laser beam to a target (hereinafter, also referred to as “LPP EUV light source apparatus”), a DPP (discharge produced plasma) light source using plasma generating by discharge, and an SR (synchrotron radiation) light source using orbital radiation. Among them, the LPP light source has the advantages that extremely high intensity close to black body radiation can be obtained because plasma density can be considerably made larger, that light emission of only the necessary waveband can be performed by selecting the target material, and that an extremely large collection solid angle of 2π steradian can be ensured because it is a point light source having substantially isotropic angle distribution and there is no structure surrounding the light source such as electrodes. Therefore, the LPP light source is thought to be predominant as a light source for EUV lithography requiring power of several tens of watts.
Here, a principle of generating EUV light in the LPP system will be explained by referring to FIG. 14 of U.S. Pat. No. 6,987,279 B2. From a target nozzle 101, a material (target material) that is excited and turned into plasma when a laser beam is applied thereto is supplied. The target material is supplied in the form of continuous flow of a liquid or gas (target get), in the form of generated droplets (droplet target), or in the form of granular solid. To the target material, a laser beam emitted from a laser unit (drive laser) 102 and collected by a focusing lens 103 is applied. Thereby, the target material is excited and plasma 104 is generated, and various wavelength components including EUV light are radiated from the plasma. On the other hand, for example, a film in which molybdenum and silicon are alternately stacked (Mo/Si multilayer film) is formed on the reflecting surface of an EUV collector mirror 105 in order to selectively reflect a predetermined wavelength component (e.g., near 13.5 nm). By the EUV collector mirror 105, the predetermined wavelength component (EUV light) radiated from the plasma 104 is reflected and collected, and outputted to an exposure unit or the like.
In the LPP EUV light source apparatus, the influence of fast ions and fast neutral particles emitted from plasma is problematic. This is because the EUV collector mirror is located near the plasma and the reflecting surface of the mirror is sputtered and damaged by those particles.
Nevertheless, the EUV collector mirror is required to have the high surface flatness of about 0.2 nm (rms), for example, in order to maintain the high reflectance, and thus, the EUV collector mirror is very expensive. Accordingly, the longer life of the EUV collector mirror is desired in view of reduction in operation costs of an EUV exposure equipment (exposure equipment using EUV light as a light source), reduction in maintenance time, and so on. The scattered materials from the plasma including fast ions and neutral particles, and the remains of the target materials are called debris.
In order to solve this problem, U.S. Pat. No. 6,987,279 B2 discloses an extreme ultra violet light source device comprising a target supply unit for supplying a material to become a target, a laser unit for generating plasma by applying a laser beam to the target, a collection optical system for collecting extreme ultra violet light radiating from the plasma and emitting the extreme ultra violet light, and magnetic field generating means for generating a magnetic field within the collection optical system when supplied with current so as to trap charged particles emitted from the plasma. That is, in U.S. Pat. No. 6,987,279 B2, the fast ions emitted from the plasma are trapped by the effect of the magnetic field, and thereby, collision with the EUV collector mirror can be prevented. Further, U.S. Pat. No. 6,987,279 B2 also discloses that the neutral particles are applied with ultraviolet light or the like to be ionized in order to trap the neutral particles having no charge in the similar way.
Further, Japanese Patent Application Publication JP-P2006-80255A discloses an extreme ultra violet light source apparatus including a chamber in which extreme ultra violet light is generated, target supply means for supplying a material to become a target into the chamber, a laser beam source for applying a laser beam to the target to generate plasma, collective optics for collecting the extreme ultra violet light radiated from the plasma, ionizing means for ionizing neutral particles included in particles emitted from the plasma into charged particles, and a magnet for forming a magnetic field within the chamber for trapping at least the neutral particles ionized by the ionizing means. Further, JP-P2006-80255A discloses that ionization of the neutral particles is performed by allowing plasma (ionization plasma) to collide with the neutral particles, and that, as a method of generating ionization plasma, electron cyclotron resonance (ECR) is caused by radiating microwave to electrons (paragraphs 0037-0040).
Typically, in the LPP EUV light source apparatus, in view of EUV conversion efficiency and so on, plasma is generated by laser oscillation in pulse operation in which the pulse width is about several nanoseconds to several tens of nanoseconds and the repetition frequency of the continuous pulse is about 1 kHz to 10 kHz. Also, in a DPP EUV light source apparatus, plasma is generated by discharge at the repetition frequency of about 1 kHz to 10 kHz. However, JP-P2006-80255A does not disclose any characteristics and generation timing of microwave to be used for causing ECR.
Here, after plasma is generated, the length of time, in which the neutral particles emitted from the plasma are scattered, is about several microseconds. Accordingly, when microwave is continuously radiated, most of the microwave energy is not used for ionization of the neutral particles by ECR, but finally emitted as thermal energy into the chamber of the EUV light source apparatus. This fact is problematic in view of effective use of energy. Further, when thermal energy is emitted into the chamber, the generation of fine target jet or droplet target is disturbed and the state of the target becomes unstable. Especially, the target liquefied by cooling a material, which is in a gas state at normal temperature, like xenon (Xe), i.e., liquefied xenon jet or liquefied xenon drop target, for example, is sensitively affected by the temperature change around. Such instability of the target causes problems of reduction in output of generated EUV light, reduction in stability of EUV pulse energy, and so on, and further, leads to reduction in exposure performance in an EUV exposure equipment and reduction in exposure processing performance.
Further, another problem caused by continuous radiation of microwave is generation of X-ray. That is, in the case where a metal material is used as the target material or a so-called mass-limited target including a droplet target is used, the residual gas pressure within the chamber becomes lower. Accordingly, the ionization of neutral particles by ECR is effectively caused only in a period as short as several microseconds in a region near the EUV plasma where particle density is relatively high. However, the electron in the ECR state then continues to absorb the microwave energy without colliding with any neutral particles. As a result, the electron obtains extremely large kinetic energy, and finally, generates radiation (X-ray) by circling motion. The X-ray generation is a serious problem because X-ray has adverse effects on human bodies and environments.